The invention relates to a seal for a chambered doctor blade of a printing machine, in the form of a rubber-elastic body, which is inserted at the end of the chambered doctor blade in a seal holder and lies against the periphery of a roller, against which the chambered doctor blade is placed.
A chambered doctor blade is used, for example, in a flexographic printing machine, for inking an anilox roller, which then, in turn, transfers the printing ink to the block of the printing cylinder. The chambered doctor blade forms a chamber, which extends in the longitudinal direction of the anilox roller, is filled with printing ink during the printing operation and is bounded on the side, facing the anilox roller, by two doctor blades, which are placed at an angle against the periphery of the anilox roller. The seals, which form the object of the invention, are intended to close off the chamber tightly at both ends. Consequently, the seal must lie against the peripheral surface of the rotating anilox roller and is consequently exposed to appreciable mechanical stresses as well as to much wear. The stiffer the seal and the greater the force, with which it is pressed against the anilox roller, the higher is the sealing action and the greater is also the wear resulting from the friction of the anilox roller. Conversely, if the seal is relatively soft, leaks can occur not only at the contact line between the anilox roller and the seal, but also between the seal and the seal holder. Furthermore, the sealing problem is made more difficult owing to the fact that, during the operation of the printing machine, there is wear of the doctor blades, so that the geometry of the cross section, which is to be sealed, is changed. The places, at which the seal, the anilox roller and the doctor blade adjoin one another, is particularly susceptible to leakage.
From the art, a seal of the type mentioned above is known, which has a continuous lip, set at an angle, on the side, facing the anilox roller, as well as on the side, facing the seal holder. The compliance of the lip can be adjusted by the inclination and profile of the lip, so that a suitable compromise is achieved between sealing properties, wear susceptibility and tolerance equalization.
It is an object of the invention, to improve the sealing properties of such a seal.
It is an object of the invention, to improve the sealing properties of such a seal. Pursuant to the invention, this objective is accomplished owing to the fact that the part of the seal, taken up in the seal holder, is constructed as a solid, rectangular support, which lies, with one end face, with a smooth surface, in contact with the chambered doctor blade and is surrounded at the three remaining sides by continuous tolerance equalization lips.
The tolerance equalization lips compensate for the clearance between the sealing body and the seal holder. Due to frictional forces in the direction of rotation, the rotating anilox roller has a tendency to carry along the seal, so that the latter is pressed firmly against the wall in question of the chambered doctor blade. At this place, the full-surface contact of the seal has the advantage that the supporting forces can be absorbed stably, so that the properties of the seal are not affected by fictional displacements and/or deformations of the seal. At the three remaining sides, at which the tolerance equalization lips are formed, the seal, on the other hand, is softer, so that it can be inserted without problems in the seal holder and nevertheless, over the whole of the periphery, ensure good sealing at the inner surface of the seal holder.
Advantageous developments of the inventions arise out of the dependent claims.
Preferably, at least at the end face, the seal has a rib, which extends at right angles to the tolerance equalization lips there, blocks the grooves formed between the tolerance equalization lips and thus prevents the flow of printing ink around the seal into these grooves. On the opposite end face, on which the whole face of the seal lies against the chambered doctor blade, a corresponding rib can be formed by the joint seam, which is formed anyhow when the seal is produced with the help of a two-part mold. In this way, reliable sealing is ensured here also.
In a preferred embodiment, the seal has two lips, which together form a V-shaped cross section and lie against the peripheral surface of the roller. A redundancy and, with that, an improvement in the sealing properties is achieved by the two lips. In addition, the V-shaped configuration causes the lips to contact the roller at an angle in opposite directions, so that the sensitivity with respect to changing mechanical stresses is reduced appreciably.
In practice, the seal frequently is exposed to a changing pressure gradient between the interior of the chambered doctor blade and the surroundings. During the printing operation, there is generally an overpressure in the chambered doctor blade. On the other hand, while the inking system is being cleaned and the chambered doctor blade is being flushed with a cleaning fluid, there is frequently a reduced pressure in the chamber. In the case of the inventive seal, these changing pressure stresses always have the effect that one of the lips is swiveled away from the anilox roller, so that its sealing action is reduced. However, to compensate for this, the other lip is pressed all the more tightly against the anilox roller. In this way, a high sealing effect is achieved, independently of the direction of the pressure gradient. Because of this effect, the seal is also less sensitive to changing stresses, which can come about, for example, due to an axial clearance of the anilox roller and/or the thermal expansion of the anilox roller.
Preferably, the lips are formed not only in the part of the seal, which is in contact with the anilox roller, but also in the parts, which are in contact with the doctor blade. A high compliance of the seal is achieved in this way, particularly at the points, at which the peripheral surface of the anilox roller, the sealing lip and one of the doctor blades coincide, so that the sealing lip can also adapt itself well to any wear at the doctor blade.
In a particularly preferred embodiment, the contact lines, made by the two sealing lips with the peripheral surface of the anilox roller, run parallel to one another, whereas the contact lines in the region of the doctor blade diverge obliquely to the outside. The sealing lips are under a slight pressure at the periphery of the anilox roller and at the doctor blades and, as a result, are bent apart slightly. If there is wear at the edge of the doctor blade, the obliquely diverging parts of the contact lines of the sealing lips with the anilox roller come into frictional contact and the frictional forces ensure that the sealing lips in this region are restored somewhat to the upright position once again in this region. In this way, good contact of the sealing lip with the anilox roller and with the doctor blade is achieved especially at the point, at which the peripheral surface of the rotating anilox roller runs out at an acute angle onto the doctor blade. Good contact of the sealing lip is particularly important especially at this point, because the printing ink is carried along by the rotating anilox roller and pushes against the edge of the doctor blade, so that good sealing against the dynamic pressure of the printing ink is required at this point.
The above-described course of the contact lines of the sealing lips at the anilox roller and at the doctor blades is achieved preferably owing to the fact that the seal, in its part facing the anilox roller, tapers in pyramid fashion towards the anilox roller. At the same time, this has the advantage that the parts of the sealing lips lying in contact with the anilox roller can be bent apart more or less, depending on the contacting force, without coming up against the walls of the seal holder.
At the inner sides, the two sealing lips preferably are bounded by a V-shaped notch, which passes in the peripheral direction of the anilox roller through the seal. Since the surface regions of the seal, on which the two doctor blades rest, brace obliquely from the periphery of the anilox roller, the notch in these regions runs out into an acute angle, so that the cross section of the sealing lips increases progressively from here to the ends of the seal. In this way, the stiffness of the sealing lips is controlled so that the latter, are relatively compliant in the region of the anilox roller and at the transitions between the anilox roller and the doctor blades and become stiffer in the regions, which support the doctor blades, so that a firm contact with the stationery doctor blades is achieved.
At the outside of the seal, the sealing lips are bounded preferably by pockets, which are constructed in the flanks of the tapering part of the seal and also extend into the regions, in which the seal is in contact with the doctor blades. The stiffness of the sealing lips can be adjusted, as required, by means of the geometry of these pockets.